Such methods are used for testing rods, wires or plates.
One known method for non-destructive testing of materials is the eddy current method. In this method defects in materials are detected by the production of a magnetic field by induction. This method is also used in wire testing equipment.
It is known, for example, to use rotary testing equipment for testing wire. In testing wire with rotary testing equipment a free ultrasonic wave is produced in the material, the reflection echo of which at the defect in the material can be detected.
However, external defects, which make up the greater part of the defects in materials, cannot be detected by this method. Moreover, free waves cannot be produced in all materials. Hence, for example, material testing using a free wave is not possible in wires having a diameter less than 15 mm.
A known ultrasonic testing method makes use of a one-dimensional wave (mode) which is generated by means of a piezoelectric transducer. By a one- or two-dimensionally guided ultrasonic wave is meant an elastic wave of which the wavelengths in one or two dimensions respectively are comparable to or large compared with the linear dimensions of the body being measured. Parts of the boundary surface are constantly in interaction with the wave and thus cause guidance of the wave along this boundary surface. The body thereby acts as a wave guide.
The reflection echo of the wave can be detected and then provides information about the existence of a defect in the material. From the amplitude of the echo some evidence of the size of the defect can also be derived. This evidence is however based on experiment and estimated values and frequently does not agree with the actual facts. False estimates are often attributable to spurious echoes, for example the echo from the outer wall of the material or echoes from other sound reflectors.
From German published patent application 41 33 648 it is known to test rods with two-dimensionally guided bar waves. This enables sensitive investigation of defects and determination of their size to be performed.
By evaluation of the amplitude, the course and the damping of the wave reflection at the defect a cross-sectional area of the defect can be calculated by means of the equivalent defect method. While the measured reflection value includes implicit information about the size and type of the defect, it is not possible to gain information about the type of defect, i.e. the eccentricity of the defect, since there is no known way of forming a relationship between the measured values and the eccentricity of the defect. All attempts to classify types of defect have therefore failed.
In testing strip using one-dimensionally guided plate waves (“Lamb modes”) the applicant has already attempted, by the use of a system with a guided wave which is beamed into the test piece using two angles of incidence, to derive the eccentricity of the defect in the material from the two echoes which are detected. However, these attempts have not yielded any success because of the concrete properties of the material.